Method And System For Treating A Fibrous Suspension

ABSTRACT

Methods for treating a fibrous suspension are disclosed, including cleaning the fibrous suspension in a screening room with a plurality of cleaning stages and including a final cleaning stage, and in which the cleaning takes place at a low fiber concentration, and recycling at least a partial flow of the accept from the last cleaning stage to a position before or in the screening room after it passes through a thickener, where the concentration of the accept is increased to a concentration which minimizes dilution of the concentration of the fiber suspension, to which the accept is supplied.

This invention relates to a method for treating a fibrous suspension, more precisely, a method for treating an accept from a screening stage, and a system for carrying out the treatment method. The method and the system are intended to be used as a stage in the pulp manufacture of a lignocellulosic material.

At the screening of a fibrous suspension it is desired to separate different types of impurities contained in the fibrous suspension. Examples of undesirable impurities are undigested chip bits, knots, shives, bark and sand. The screened fibrous suspension has to meet different purity requirements, depending a.o. on the type of paper to be manufactured. The requirements for flax pulp, for example, are lower than for bleached pulp. In order to achieve an expedient screening of a fibrous suspension, the screening room often comprises a plurality of stages, because no cleaning arrangement operates with such perfection, that it can separate all impurities in one stage. The cleaning arrangements comprised in the screening room can be of several different types, such as screens of various types and/or vortex cleaner and cyclones.

Effective operation of a screen often requires to take out a relatively large reject flow. A large reject flow implies at the same time that a large portion of the fibers desired to be retained as accept will follow along. In order to recycle these fibers, and thereby to reduce fiber losses, cascade connection often is used between the cleaning arrangements comprised in the screening room. This means that the reject from a first screen is moved to a subsequent screen where it is screened. The accept from the subsequent screen is then recycled to the inject before the previous screen. By such cascade connections a large portion of the fibers desired to remain in the process is recovered.

The requirements on minimizing fiber losses become increasingly higher. In order to additionally reduce fiber losses, the last cleaning arrangement in the form of, for example, a final stage screening or knot-washing is important. The last cleaning arrangement can also be cyclones or vortex cleaner. The last cleaning stage is carried out at relatively low fiber concentration for achieving an effective separation. Such a last cleaning stage typically is operated at fiber concentrations of the magnitude 1%. The accept from such a cleaning arrangement has a concentration of the magnitude 0.5%. The accept from the last cleaning stage can be recycled to the process to a mixing tank located before the screening room. The mixing tank can be, for example, a storage tank or a blow tank.

One problem at the recycling of a fibrous suspension of low fiber concentration from a last cleaning stage is, that the recycling can cause the fiber concentration to get lower in the entire screening room. At too low a fiber concentration problems arise with limitations of hydraulic capacity, above all in subsequent stages, such as, for example, washing equipment. The term hydraulic capacity refers to the size of the liquid amount which per time unit can pass through an equipment. A low fiber concentration implies, seen relatively, greater amounts of liquid, and the capacity of the equipment then is restricted to the possibility for the liquid to get into and through the equipment in question. At a large liquid amount, besides, the fibrous suspension can be caused to be forced through the screening room. Such an increased passing speed brings about an inferior screening result. A subsequent washing equipment can also have a worse washing result.

At present it is difficult to cascade connect, i.e. to recycle the accept from a last cleaning arrangement, which is operated at low fiber concentration, to the next previous cleaning arrangement, because the liquid volumes obtained at the lower fiber concentration become so large, that a cascade connection practically is impossible. The only possible recycling position will be a mixing tank with high volume capacity.

The present invention has the object to solve the problems with capacity limitation in a treatment line for fibrous suspension.

A further object is to overcome the problems with liquid balances in a system for the treatment of a fibrous suspension.

Still another object is to render possible cascade connection from a last cleaning arrangement operated at low fiber concentration.

According to the invention, the system consists of a screening room, which comprises a plurality of cleaning arrangements. The cleaning arrangements can be, for example, parallel connected or cascade connected. The design of the screening room depends a.o. on which production volumes it is intended to deal with. According to the invention, a thickener is placed in the process line, so that at least a partial flow of the accept from a last cleaning arrangement, operated with low fiber concentration, passes through the thickener before the accept is recycled to a mixing tank located before the screening room, alternatively to one of the screening stages comprised in the screening room. The accept preferably passes through an intermediate storage tank located in a position before the thickening stage prior to that at least a partial flow of the accept passes through the thickener.

The method according to the invention comprises the feature to allow at least a partial flow of the accept from a cleaning arrangement operated at low fiber concentration to pass through a thickener, before the fraction is recycled to a position located before or in the screening room.

At an embodiment of the method according to the invention, the concentration of the fibrous suspension in the thickener is increased suitably to a concentration, which minimizes a lowering of the concentration in the mixing tank. According to an embodiment of the method, the concentration in the partial flow passing through the thickener is increased to an interval of 1%-5%. The thickener can be any kind of equipment concentrating a fibrous suspension.

The invention is described in greater detail in the following, with reference to the Figures illustrating different embodiments of the invention.

FIG. 1 shows a process layout of a system according to the invention. From a mixing tank (1) a fibrous suspension with a concentration of 1-5% is moved through a screening room (2). The screening room comprises a plurality of screening stages (21,22,23), which at this embodiment are parallel connected without recycling. The accept from the screening stages is moved to a subsequent washing equipment (5). The reject from the screening room is moved to a cleaning arrangement (3), which is operated at low fiber concentration (magnitude 1%). The accept leaving the cleaning arrangement has a concentration of the magnitude 0.5%. From the cleaning arrangement (3) the accept is moved to an intermediate storage tank (6). After the intermediate storage tank a partial flow of the accept passes through a thickener (4). The thickener is located in a by-pass (9) to the main line (8). The filtrate from the thickener is moved to a filtrate tank (7), to which also the filtrate from the washing equipment (5) is moved. The thickened fibrous suspension is recycled to the mixing tank (1).

FIG. 2 shows a process layout of a system according to another embodiment of the invention. At the embodiment shown the screening room consists of three cascade connected screens (24,25,26). The accept from the third screen (26) is recycled to the second screen (25), and the accept from the second screen (25) is recycled to the first screen (24). The reject from the last screen (26) is moved to a cleaning arrangement (3). The accept from the cleaning arrangement (3) is moved via an intermediate storage tank (6) to a thickener (4). The thickened fibrous suspension is recycled to the intermediate storage tank (6). From the intermediate storage tank the thickened fibrous suspension is recycled to the third screen (26). The filtrate from the thickener is moved to the filtrate tank (7) of the washing equipment.

The invention, of course, is not restricted to the embodiments shown, but can be varied within the scope of the claims. 

1-5. (canceled)
 6. A method of treating a fibrous suspension comprising cleaning said fibrous suspension in a screening room including a plurality of cleaning stages including a final cleaning stage, carrying out said cleaning of said fibrous suspension in said final cleaning stage at a low fiber concentration so as to produce an accept portion and a reject portion of said fibrous suspension, passing at least a portion of said accept portion of said fibrous suspension from said final cleaning stage to a thickener to produce a thickened flow of said fibrous suspension, and recycling said thickened flow of said fibrous suspension to at least one cleaning stage prior to said final cleaning stage, whereby the dilution of said fibrous suspension in said screening room is minimized.
 7. The method of claim 6 wherein said plurality of cleaning stages includes a plurality of cleaning stages prior to said final cleaning stage.
 8. The method of claim 6 wherein said thickened flow of said fibrous suspension is recycled to a mixing tank prior to said screening room.
 9. The method of claim 6 including passing at least another portion of said accept portion from said final cleaning stage in a bypass line past said thickener.
 10. The method of claim 6 wherein said final cleaning stage comprises apparatus selected from the group consisting of a final stage screen, a knot washer, a vortex cleaner, and a cyclone.
 11. The method of claim 6 wherein said low fiber concentration comprises a fiber concentration of about 1% or less. 